This invention relates to test and measurement instruments and, more particularly, to methods and apparatus for receiving an optical signal and transmitting the received optical signal to an optoelectrical transducer for photodetection. Specifically, the invention is directed to a gradient-index (GRIN) rod lens connected in an optical circuit between an optical launch, such as a fiber optic cable, and a photodetector for efficiently coupling light received through the optical fiber to the photodetector.
Optical return loss (ORL) is defined by: EQU ORL=10*log (P.sub.i /P.sub.r) (dB) Eq. (1)
where P.sub.i is the incident optical power and P.sub.r is the reflected optical power. In an optical receiver, high ORL is desirable for various reasons.
For example, high ORL is desirable in an optical receiver to avoid perturbing active optical components, such as distributed feedback lasers used as optical sources. Agrawal, G. P., and Dutta, N. K., Long Wavelength Semiconductor Lasers, (Van Nostrand Reinhold, New York, 1986); Tkach, R. W., and Chraplyvy, A. R., "Linewidth broadening and mode splitting due to weak feedback in single-frequency 1.5 .mu.m lasers," Elect. Lett. 21, 1081-1083 (1985). High ORL is also desirable to reduce measurement mismatch uncertainty when the optical receiver is used in a lightwave component measurement system that measures modulation bandwidth.
One known optical receiver comprises a single-mode optical fiber, a GRIN rod lens, and a photodetector connected in an optical circuit. For example, the GRIN rod lens can be a 0.2 to 0.3 pitch GRIN rod lens in a 1:1 or magnified imaging position with respect to the photodetector. In this known optical receiver, optical reflections can occur at the optical fiber end face, both faces of the GRIN rod lens, and the face of the photodetector.
To achieve a high ORL, reflections from these faces must either be minimized or deflected away from the optical return path through the optical fiber. Specular reflections from the optical fiber end face are conventionally deflected away from the optical return path. Marcuse, D., "Reflection losses from imperfectly broken fiber ends," Appl. Opt. 14, 3016-3020 (1975); Ulrich, R., and Rashleigh, S. C., "Beam-to-fiber coupling with low standing wave ratio," Appl. Opt. 19, 2453-2456 (1980). Such reflections from the lens entrance face are also traditionally deflected away from the optical return path. von Bally, G., Schmidthaus, W., Sakowski, H., and Mette, W., "Gradient-index optical systems in holographic endoscopy," Appl. Opt. 23, 1725-1729 (1984). Specular reflections from the face of the photodetector are also commonly deflected away from the optical return path. This is typically achieved by appropriately bevelling the optical fiber end face, as well as the lens entrance face and, additionally, by tilting the photodetector with respect to normal incidence on the face of the photodetector.
Furthermore, reflections from the lens exit face are typically minimized with an anti-reflection (AR) coating. However, one drawback is that the ORL bandwidth of the optical receiver is undesirably limited by the AR coating bandwidth.
It is therefore desirable to increase the optical return loss of an optical receiver. Furthermore, it is desirable to achieve higher optical return loss across a wide range of wavelengths.